Field of the Invention
The invention relates to a configuration for decomposing nitrogen oxides in a gas stream which includes a plurality of catalytic converters disposed one after the other through which the gas can flow. Each of the catalytic converters has a honeycomb structure with many parallel cells through which the gas stream can flow and which are present at a particular cell density. Each of the catalytic converters also has an average level of activity defined as the proportion by weight of the catalytically active agent in the catalytic converter. A configuration of this type is disclosed in Published European Patent Application EP 0 611 594 A1, Published, Non-Prosecuted German Patent Application DE 40 27 329 A1, U.S. Pat. No. 4,118,199 or German Utility Model DE 90 12 384 U1.
A configuration of this type includes, in particular, catalytic converters which decompose nitrogen oxides by a selective catalytic reduction (SCR) process. For which purpose a suitable reducing agent must be added to the gas stream. Ammonia is preferably used as the reducing agent. In order to add ammonia to the gas stream, either ammonia itself, or a compound which readily decomposes and releases ammonia, in particular, urea can be distributed in the gas stream.
A configuration of this type is preferably a component of an exhaust gas system of a combustion drive unit, for example a diesel engine or a gas turbine, and the gas stream is an exhaust gas given off by the combustion drive unit. In principle, the configuration is used on any combustion drive unit which gives off an exhaust gas which still includes oxygen. It can thus also be a petrol or gas engine which is operated with excess air.
From the prior art publications cited in the introduction, it follows that the catalytic converters through which the gas stream flows one after the other are furnished with different chemical properties, see, in particular, Published European Patent Application EP 0 611 594 A1, U.S. Pat. No. 4,118,199 and Published, Non-Prosecuted German Patent Application DE 40 27 329 A1. In particular, a catalytically active substance which is a constituent of the catalytic converter can be provided at increasing concentration along the flow path of the gas stream.
According to the German Utility Model DE 90 12 384 U1, in a corresponding apparatus, catalytic converters having geometries differing from one another are disposed one after the other. These catalytic converters can, in particular, be catalytic converters of the honeycomb type. A catalytic converter of the honeycomb type is characterized in each case by the cell density of the cells through which the gas stream can flow are disposed. In general the cells are connected side by side and in parallel to one another.
It is accordingly an object of the invention to provide a configuration for decomposing nitrogen oxides in a gas stream and a method for using the configuration which overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, which can be adapted in a particular manner to the relevant parameters of the gas stream for which it is intended and which is distinguished by a particularly high efficiency with a particularly small construction.
With the foregoing and other objects in view there is provided, in accordance with the invention, a configuration for decomposing nitrogen oxides in a gas stream flow, including: a plurality of catalytic converters having a first catalytic converter and a second catalytic converter disposed one behind the other for receiving and conducting the gas stream flow, each of the catalytic converters having a honeycomb structure with a multiplicity of parallel cells through which the gas stream flows, each of the catalytic converters also having a predetermined cell density of the parallel cells and a predetermined average level of activity defined by a percentage of weight of a catalytically active agent in a respective catalytic converter, the predetermined cell density of the second catalytic converter being lower than the predetermined cell density of the first catalytic converter, and the predetermined average level of activity of the second catalytic converter being higher than the predetermined average level of activity of the first catalytic converter.
To achieve the object of the invention, the cell density of the second catalytic converter through which the gas stream can flow downstream of the first catalytic converter is lower than the cell density of the first catalytic converter, and the average level of activity of the second catalytic converter is higher than the average level of activity of the first catalytic converter.
Here, catalytic converters are therefore disposed one after the other which are considerably different from one another both with respect to their chemical composition and also with respect to their geometry. Preferably, catalytic converters are present in a multiplicity, for example four catalytic converters, and the properties mentioned change from catalytic converter to catalytic converter preferably monotonically. This provides additional degrees of freedom for construction of the configuration and makes it possible for the configuration to be adapted considerably better than hitherto to the requirements resulting from given properties of the gas stream.
In particular, it is possible to construct the configuration in such a manner that it is fully active in a broad temperature range, for example from 170xc2x0 C. to 500xc2x0 C. For this purpose, the catalytic converters in the configuration can be constructed in such a manner that they develop their respective highest efficiency at respective maximum temperatures which are different from one another. It is also possible to take into account an interaction which arises with respect to the activity between the catalytic converters, in particular, the geometry of the second catalytic converter can take into account specific properties of the gas stream which arise from the fact that it has already flowed through the first catalytic converter. Furthermore, the selectivity of the configuration can be considerably improved in comparison with the prior art with respect to the reactions to be catalyzed by the configuration, since, with appropriate account being taken of the interactions occurring between the catalytic converters. In this manner, any catalytic converter can be optimized to a particularly selectivity with respect to the prevailing conditions.
The conversion of pollutants in the exhaust gas which flows through the configuration of the catalytic converter remains small initially, since the activity of the first catalytic converter is relatively small. This is preferably caused by the catalytic converter having a relatively low content of the catalytically active agent, for example divanadium pentoxide. This certainly makes the catalytic converter highly suitable for reacting pollutants in an exhaust gas which has a relatively high temperature, which, in the case of the first catalytic converter of the configuration, will generally be true. The second catalytic converter has a relatively high activity and is thus highly suitable for removing from the exhaust gas pollutant residues remaining downstream of the first catalytic converter. The relatively high content of the catalytically active agent in the second catalytic converter makes this particularly suitable, especially when the catalytically active agent is divanadium pentoxide, for removing pollutants from exhaust gases having relatively low temperatures, as is generally the case at the second catalytic converter. Generally, it can be expected that the temperature of the exhaust gas decreases during its flow through the configuration. The distribution of activity over the catalytic converters thus particularly complies with the specific properties of the catalytically active agent.
The choice of a comparatively high cell density for the first catalytic converter and of a comparatively low cell density for the second catalytic converter takes into account the distribution of activity over the configuration in a particularly advantageous manner. This is because the choice of a relatively high cell density for the first catalytic converter makes up for low activity of the catalytic converter by the fact that it provides in the first catalytic converter a particularly high surface area on which the reaction to be catalyzed can take place. In this manner, even at a comparatively low activity, a good conversion rate of the pollutants present in the exhaust gas can be achieved. The choice of a high activity for the second catalytic converter, in contrast, permits the choice of a comparatively low cell density and thus of a comparatively low surface area in the second catalytic converter for the reaction to be catalyzed, and thus permits a decrease of the pressure drop in the exhaust gas stream caused by introducing the configuration. The present configuration thus complies in a particularly advantageous manner with the flow conditions and chemical conditions which must be heeded in the removal of pollutants from an exhaust gas stream.
In the context of a preferred development of the configuration, a front most catalytic converter, through which the gas stream can flow first, has a cell density of 200 cpsi to 400 cpsi. A rear most catalytic converter through which the gas stream can flow last has here a cell density of 10 cpsi to 100 cpsi. The unit xe2x80x9ccpsixe2x80x9d has established itself as a commonly used unit in the subject, defined as xe2x80x9ccells per square inchxe2x80x9d. The cell density of the front most catalytic converter can be varied independently of the cell density of the rear catalytic converter and can be adapted to the requirements of the particular application. It is just as possible to choose the cell density of the front most catalytic converter at 400 cpsi and the cell density of the rear most catalytic converter at 10 cpsi or to provide the front most catalytic converter with 200 cpsi and the rear catalytic converter with 100 cpsi. In order to put the citation of cell density in proportion to the catalytic converter dimensions used in a concrete application, it may be noted that a conventional catalytic converter, measured in a cross section, has a diameter or a thickness of at least 2 inches or 5 cm. Thus a conventional catalytic converter having a cell density of 10 cpsi possesses a good 100 cells or more.
It is further preferred that every catalytic converter in the configuration includes a ceramic material based on titanium dioxide and/or tungsten trioxide. An especially effective catalytically active agent used in this connection is divanadium pentoxide. The average level of activity, measured by the content of divanadium pentoxide, is preferably between 0% and 5% for the front most catalytic converter and between 15% and 20% for the rear most catalytic converter.
Further preferably, the catalytically active agent is inhomogeneously distributed in every catalytic converter. The concentration of the catalytically active agent in the catalytic converter increasing from the inlet side to the outlet side in the direction of flow determined by the gas stream. With regard to the remark made in the preceding paragraph, in which the catalytically active agent is divanadium pentoxide, its concentration can increase monotonically from 0% to 20% from the front most to the rear most catalytic converter. Possible methods of producing an inhomogeneous concentration of this type are known from the prior art and do not require any further explanation here.
An injection unit for injecting a reducing agent, preferably a urea, into the gas stream upstream of the catalytic converters is preferably assigned to the configuration. As already explained, the configuration can be adapted particularly well to a gas stream which has parameters which vary greatly during operations. Even in this case a particularly compact construction of the configuration is possible. The configuration is thus particularly suitable for use in a mobile unit, for example in a motor vehicle engine. For such an application, in particular, the use of urea as the reducing agent is preferred, since urea, in contrast to pure ammonia, can be provided in the form of a stable and not excessively chemically reactive aqueous solution. The configuration is thus most suitable for applications in which lay persons must occasionally supplement a store of the reducing agent.
With the foregoing and other objects in view there is also provided, in accordance with the invention, a method for decomposing nitrogen oxides in a gas stream flow, which includes: placing a plurality of catalytic converters having a first catalytic converter and a second catalytic converter one behind the other, each of the catalytic converters having a honeycomb structure with a multiplicity of parallel cells, each of the catalytic converters also having a predetermined cell density of the parallel cells and a predetermined average level of activity defined by a percentage of weight of a catalytically active agent in a respective catalytic converter, the predetermined cell density of the second catalytic converter being lower than the predetermined cell density of the first catalytic converter, and the predetermined average level of activity of the second catalytic converter being higher than the predetermined average level of activity of the first catalytic converter; and processing a gas stream discharged as exhaust gas from a combustion drive unit through the plurality of catalytic converters for decomposing nitrogen oxides in the gas stream, the gas stream having a temperature fluctuating between about 170xc2x0 C. and about 500xc2x0 C.
According to the invention, the use of the configuration is envisaged for the decomposition of nitrogen oxides in a gas stream which is discharged as exhaust gas from a combustion drive unit, the gas stream having a temperature fluctuating between about 170xc2x0 C. and about 500xc2x0 C.
Particularly preferably here, urea, in particular in aqueous solution, is injected into the gas stream upstream of the catalytic converters.
Particular preference is also given to the use of the configuration in which the catalytic converters decompose the nitrogen oxides by a process of selective catalytic reduction.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a configuration for decomposing nitrogen oxides in a gas stream and a method for using the configuration, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.